With increased emphasis in the food industry for modification of food ingredients to meet legal requirements and consumer demands, for purpose of reducing costs of new and existing products and for purpose of nutritional fortification of foods, new sources of functional proteins are emerging.
Functional properties of proteins which are of interest in foods include solubility, emulsification, foaming or whipping, water binding, fat binding, gelation, viscosity, thickening, adhesion, cohesion and flavor. Of utmost importance is the production of hydrolyzates with no off-flavors or bitterness.
During the past decade utilization of plant proteins, especially from soybeans, has increased tremendously, primarily for nutritional and economic reasons. In many cases, however, the texture or flavor needs to be altered for certain uses. Functionally modified proteins, collectively known as hydrolyzed proteins or hydrolyzates, have been introduced quite recently to meet this need.
The usual raw materials for manufacturing hydrolyzates are meat, fish, blood, dairy products, grains, alfalfa and other leaves, and oilseeds such as soybeans, peanuts, and cottonseed. Either chemical (acid or alkali) or enzymatic methods can be used to produce hydrolyzates. In acid hydrolysis, strong acids at high temperatures break the chemical bonds of the protein. Uncomplicated but relatively harsh, this treatment can result in some loss of essential amino acids and in undesirable side-reactions with non-protein components of the reaction mixture. Alkaline hydrolysis also requires fairly extreme conditions for producing the reaction. Consequently there is always the danger that lysinoalanine, a potentially toxic by-product, may form. In addition, the large amount of residual acid or alkali in the hydrolyzate limits its use in most food products.
Enzyme hydrolysis is an attractive alternative to chemical treatment because the process is mild. Moreover, the inherent specificity of various proteolytic enzymes should control the nature and extent of hydrolysis and thus the functional properties of the product.
Two major problems associated with this method have so far limited its general use. First, the cost of enzymes in conventional, batch-type hydrolytic systems can be prohibitive. The protein source and the enzyme are typically mixed in suspension at the optimum temperature and pH for a few hours. When the desired degree of hydrolysis is obtained, the enzyme is inactivated either by changing the pH, increasing the temperature, or both. Hence, the enzyme can be used only once. The heat treatment also adds to the cost of this method.
Second, the extent of the reaction must be carefully controlled. Studies to date indicate that if hydrolysis goes on too long, or is controlled, off-flavors or bitterness may develop.
The bitterness, which arises from the production of small peptides, seems to be especially pronounced if peptides with a low molecular weight are produced. However, this condition depends to some extent on the protein and on the specificity of the enzyme. Milk and soy proteins, in particular, often develop an intensely bitter flavor when hydrolyzed.
Many of the unwanted effects can be overcome by using enzyme immobilization or ultrafiltration, newer technologies that are developing rapidly. In the first of these, the enzyme is immobilized, either by chemical procedures or physical adsorption, when attached to a solid support such as silica, alumina, or iron oxide. The reaction mixture is then allowed to flow through a column containing the immobilized enzyme. The extent of hydrolysis is controlled essentially by the flow rate or length of time in the reactor. A major drawback is that immobilization causes a large drop in enzyme activity. The procedure is also fairly expensive.
Ultrafiltration employs membranes, which are essentially filters with very fine pores, that retain macromolecules but permit passage of small molecules.
Enzyme-membrane reactors have been demonstrated for hydrolysis of starch, and alfalfa, cottonseed, and fish proteins. In these studies, however, several problems were encountered, such as a rapid drop in reactor output from the accumulation of unhydrolyzed material on the membrane.
Earlier disclosures, as, for example, in U.S. Pat. Nos. 2,489,208; 3,713,843; and 3,830,942, have suggested that enzymatic proteolysis should be conducted at a pH comparable to the pH of end product use. Thus, for acidic products, such as low-acid beverages, where a highly soluble ingredient is needed, the hydrolysis should be performed at an acidic pH or at the iso-electric point of the protein. Problems arise, however, during neutralization of the acid hydrolyzates due to formation of salts, with accompanying salty taste and precipitation when added to acid beverages.
Large-scale production of soybean protein hydrolyzate has been conducted, employing a laminar flow, modular membrane of cellulose acetate for separation of reaction products. Hydrolysis was conducted under acidic conditions, at pH 3.7, and high temperature, 60.degree. C. Hydrolysis under these conditions leads to reduction in nutritional quality of reaction products by destruction of labile amino acids such as tryptophane and methiomine.
One application of enzymatically hydrolyzed proteins is in "defined formula" diets, or "medical foods", for consumption by those unable to properly digest or absorb whole protein. In clinical cases of severe pancreatic enzyme insufficiency or malabsorption, it has been postulated that amino acids are better absorbed from hydrolyzed protein than from the intact protein. The primary source of such pre-digested protein today is casein, which has drawbacks such as poor palatability and high cost.
Considerable attention has also been directed towards producing an acid-soluble hydrolyzate for incorporation into acidic beverages for nutritional fortification. However, preparing a completely acid-soluble and clear protein requires excessive hydrolysis which is generally accompanied by formation of bitter flavor in the product.
There remains a need for significantly improved technology to provide effective and economic means for the production of such hydrolyzates in such a manner as to retain nutritional values while overcoming solubility and flavor problems associated with existing products.